U01.01.098 Disorders of fructose metabolism

Fructose metabolism disorders are inherited conditions that affect the breakdown of fructose, leading to various symptoms and complications. Understanding these conditions is crucial for recognizing their presentation and treatment options.

1. Essential Fructosuria

Pathophysiology:

Defect: Fructokinase deficiency (autosomal recessive).
Mechanism: Fructose is not trapped inside cells, as it cannot be phosphorylated by fructokinase.
Compensatory Mechanism: Hexokinase becomes the primary pathway for converting fructose into fructose-6-phosphate.

Symptoms:

Asymptomatic or very mild.
Fructose appears in blood and urine.
No significant tissue damage due to the lack of fructose accumulation.

Key Points:

Benign and asymptomatic.
Milder than disorders of galactose metabolism.

2. Hereditary Fructose Intolerance

Pathophysiology:

Defect: Aldolase B deficiency (autosomal recessive).
Mechanism: Accumulation of fructose-1-phosphate leads to a decrease in available phosphate. This inhibits glycogenolysis and gluconeogenesis.
Symptoms: Symptoms are triggered by the consumption of fructose-containing foods (e.g., fruit, juice, honey).

Symptoms:

Hypoglycemia.
Jaundice.
Cirrhosis.
Vomiting.

Diagnosis:

Urine dipstick will be negative (glucose tests only).
Reducing sugar can be detected in urine (nonspecific for inborn errors of carbohydrate metabolism).

Treatment:

Eliminate fructose, sucrose (glucose + fructose), and sorbitol (which is metabolized to fructose) from the diet.

Fructose Metabolism Pathway Overview:

Enzyme	Substrate	Product	Effect of Deficiency
Fructokinase	Fructose	Fructose-1-phosphate	Essential Fructosuria
Aldolase B	Fructose-1-phosphate	Glyceraldehyde, Dihydroxyacetone-P	Hereditary Fructose Intolerance
Triose kinase	Glyceraldehyde	Glyceraldehyde-3-phosphate	Inhibited by lack of phosphate

Points to Remember:

Essential Fructosuria is typically benign with no major clinical effects.
Hereditary Fructose Intolerance is more serious, leading to hypoglycemia and liver dysfunction, triggered by fructose consumption.
Diagnosis can be difficult without specialized tests like reducing sugars in urine.
Both conditions are autosomal recessive.
Dietary management is the mainstay of treatment for Hereditary Fructose Intolerance.

These disorders are essential for understanding inborn errors of metabolism and their clinical implications, particularly for the USMLE Step 1.

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USMLE Step 1 Review Series

Curriculum

U01.01.098 Disorders of fructose metabolism

1. Essential Fructosuria

Pathophysiology:

Symptoms:

Key Points:

2. Hereditary Fructose Intolerance

Pathophysiology:

Symptoms:

Diagnosis:

Treatment:

Fructose Metabolism Pathway Overview:

Points to Remember:

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